This application claims priority to Japanese Patent Application Number JP2001-368020 filed Dec. 3, 2001, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a liquid discharge device, and a method of manufacturing the same. Particularly, the present invention relates to a liquid discharge device in a system in which droplets are ejected by heating with a heating element. In the present invention, in order to effectively avoid deterioration in reliability due to damage to a protective layer, an anti-cavitation layer is formed after heat treatment for stabilizing connections.
2. Description of the Related Art
In the field of image processing, needs for coloring of hard copies have recently increased. In order to meet the needs, color hard copy systems such as a sublimation thermal transfer system, a melting thermal transfer system, a liquid discharge system (ink jet system), an electrophotographic system, a thermally processed silver system, etc. have been conventionally proposed.
Of these systems, in the ink jet system as the liquid discharge system, droplets of a liquid (ink) are ejected from nozzles provided on a recording head, and adhered to a recording object to form dots, thereby outputting an image of high quality by a simple structure. This ink jet system is classified into an electrostatic system, a continuous vibration generating system (piezo system), a thermal system, etc. according to ink ejection systems.
Of these systems, the thermal system is a system in which bubbles are produced by locally heating an ink, and the ink is ejected from the nozzles by the bubbles, and flies to the recording object so that a color image can be printed by a simple structure.
Namely, this thermal-system liquid discharge device comprises a semiconductor substrate on which heating elements for heating an ink, driving circuits comprising logic integrated circuits for driving the heating elements, etc. are mounted. Therefore, the heating elements are arranged with a high density so that they can be securely driven.
Namely, in order to obtain a high-quality print result of the thermal-system liquid discharge device, the heating elements must be arranged with a high density. Specifically, for example, in order to obtain a print result corresponding to 600 (DPI), the heating elements must be arranged with intervals of 42.333 xcexcm. It is thus very difficult to respectively dispose driving elements for the heating elements arranged with such a high density. Therefore, in the liquid discharge device, switching transistors are formed on the semiconductor substrate, and the heating elements respectively corresponding to the switching elements are connected by an integrated circuit technique so that the switching transistors can be respectively driven by driving circuits formed on the semiconductor substrate to simply and securely drive the heating elements.
In the thermal-system liquid discharge device, when bubbles are produced in an ink by heating with the heating elements to eject the ink from nozzles by the bubbles, the bubbles disappear. Therefore, bubbling and debubbling are repeated to cause a mechanical shock due to cavitation. Also, a temperature rise by heating with the heating elements and a temperature drop are repeated within a short time (several seconds) to cause a great stress due to temperature.
Therefore, in the liquid discharge device, each of the heating elements is formed by using tantalum, tantalum nitride, tantalum aluminum, or the like, and a protecting layer composed of silicon nitride is formed on the heating elements, for improving heat resistance and insulation by the protecting layer, and for preventing direct contact between the heating elements and an ink. Furthermore, an anti-cavitation layer is formed on the protecting layer, for relieving a mechanical shock due to cavitation. The anti-cavitation layer has excellent acid resistance, and a passive film is easily formed on the surface of the anti-cavitation film. Also, the anti-cavitation film is made of tantalum with excellent heat resistance.
FIG. 7 is a sectional view showing the configuration of the vicinity of a heating element in this type of liquid discharge device of prior art. In the liquid discharge device 1, an insulating layer (SiO2), etc. are formed on a semiconductor substrate 2 on which semiconductor elements are formed, and then a heating element 3 comprising a tantalum film is formed. Furthermore, a protecting layer 4 composed of silicon nitride (Si3N4) is laminated, and a wiring pattern (Al wiring) 5 is formed for connecting the heating element 3 to a semiconductor formed on the semiconductor substrate 2. Furthermore, a protecting layer 6 composed of silicon nitride (Si3N4) is laminated, and an anti-cavitation layer 7 composed of tantalum is formed on the protecting layer 6.
The liquid discharge device 1 is further heat-treated (sintered) at 400xc2x0 C. for 60 minutes in an atmosphere of nitrogen gas (N2) containing 4% of hydrogen gas (H2) to stabilize the connections between the heating element and the wiring pattern and between wiring patterns, and compensating for silicon defects with the added hydrogen. Instead of the heat treatment in such an atmosphere, a heat treatment method in a hydrogen atmosphere is also proposed (Japanese Unexamined Patent Application Publication Nos. 7-76080 and 9-70973). Japanese Patent No. 2971473 discloses a method of heat-treating a protecting layer composed of silicon oxide formed by a bias sputtering process to decrease a residual stress in the protecting layer.
In the liquid discharge device 1, an ink chamber, an ink flow path, and a nozzle are then formed by disposing predetermined members. In the liquid discharge device 1, an ink is introduced into the ink chamber through the ink flow path, which are formed as described above, and the semiconductor element is driven to generate heat from the heating element, to locally heat the ink in the ink chamber. In the liquid discharge device 1, bubbles are produced in the ink chamber due to the heating to increase the pressure in the ink chamber, so that the ink is ejected from the nozzle, and flies to the recording object.
The protecting layer 6 has relatively low heat conductivity, and thus the thickness of the protecting layer 6 is decreased to improve heat conduction to the ink chamber, thereby effectively ejecting ink droplets. However, when the thickness of the protecting layer 6 is decreased, pinholes occur, and step coverage in a step portion at the interface between the protecting layer 6 and the wiring pattern 5 deteriorates to cause difficulties in completely isolating the heating element 3 from the ink. As a result, the wiring pattern 5 and the heating element 3 are corroded by the ink to deteriorate reliability, and the lifetime of the heating element 3 is shortened.
It is thus thought that when the protecting layer 6 is formed to a thickness of 300 nm, the occurrence of pinholes can be securely prevented, and sufficient step coverage can be secured in the step portion at the interface between the wiring pattern 5 and the protecting layer 6, thereby securing sufficient reliability.
According to experimental results, with the protecting layer 6 having a thickness of 300 nm, the occurrence of pinholes can be securely prevented, and sufficient step coverage can be secured. However, a crack B was observed in the protecting layer 6, as shown by arrow A in an enlarged partial view of FIG. 7. Like the pinholes, such a crack B allows the ink to enter the heating element 3, thereby significantly deteriorating the reliability of the printer head 1.
As a prior method for preventing the occurrence of the crack, a method of tapering the end surface of the wiring pattern 5 by wet etching during the formation of the wiring pattern 5 using an aluminum wiring material, as shown in FIGS. 8A and 8B, is proposed in, for example, Hewllet-Packard Journal, May, 1985, pp. 27-32. Namely, by tapering the end surface of the wiring pattern 5, the occurrence of a step in the protecting layer 6 formed thereon can be decreased, thereby preventing the concentration of stress and preventing the occurrence of a crack.
However, in a today""s wiring pattern, a wiring pattern material comprises aluminum containing silicon, copper, or the like added for improving the properties and lifetime of the wiring pattern, and thus tapering of the end surface of the wiring pattern by wet etching has a problem in which silicon, copper, or the like added to the pattern material remains unetched to leave the residue of silicon, copper, or the like as dust in the etched portion.
The present invention has been achieved in consideration of the above problem, and it is an object of the present invention to provide a liquid discharge device capable of effectively avoiding deterioration in reliability due to damage to a protecting layer, and a method of manufacturing the same.
In order to achieve the object, in a first aspect of the present invention, a liquid discharge device comprises a protecting layer formed on a heating element, for protecting the heating element from a liquid, and an anti-cavitation layer formed for protecting the heating element from cavitation, wherein after the protecting layer is formed, at least the connections between the heating element and a wiring pattern and between the wiring pattern and a semiconductor element are stabilized by heat treatment, and then the anti-cavitation layer is formed.
In a second aspect of the present invention, a method of manufacturing a liquid discharge device comprises forming a protecting layer on a heating element to protect the heating element from a liquid, performing heat treatment for stabilizing at least the connections between the heating element and a wiring pattern and between the wiring pattern and a semiconductor element, and then forming an anti-cavitation layer for protecting the heating element from cavitation.
The anti-cavitation layer is required to protect the heating element by relieving cavitation, and thus a material having high stress, such as tantalum (Ta), or the like is used for the anti-cavitation layer. The compressive stress of a tantalum film is 1.0 to 2.0xc3x971010 (dyne/cm2) . However, tantalum has a linear expansion coefficient of 6.5 (ppm/degree), aluminum generally applied to wiring patterns has a linear expansion coefficient of 23.6 (ppm/degree), and a protecting layer of Si3N4 formed between both materials has a linear expansion coefficient of 2.5 (ppm/degree). It is known that as in a conventional method, heat treatment after the formation of the anti-cavitation layer causes large thermal stress between these layers due to the differences between the linear expansion coefficients, and thus produces a crack in the protecting layer due to the thermal stress. However, in the liquid discharge device of the present invention, after the protecting layer is formed for protecting the heating element from a liquid, heat treatment is performed for stabilizing at least the connections between the heating element and the wiring pattern and between the wiring pattern and the semiconductor element, and then the anti-cavitation layer is formed for protecting the heating element from cavitation. Therefore, the concentration of thermal stress in the protecting layer during the heat treatment can be decreased, thereby effectively avoiding deterioration in reliability due to damage to the protecting layer.
Also, the method of manufacturing the liquid discharge device of the present invention can effectively avoid deterioration in reliability due to damage to the protecting layer.